lumbar fusion - broken screws

Has anyone else had spinal fusion hardware (titanium screws) break without obvious cause?

I had a burst fracture in L2 after a ground fall, and am fused from L1 - L3 with two rods and five screws. I just had my two-year post-op x-ray and was alarmed to find that two of the screws have broken. None of the hardware is loose, though, and I'm not in pain, so my doctor says there isn't actually a problem to fix. She says my bones are stable and I'm fine to keep climbing, but she doesn't know why the screws broke and I don't feel like I've experienced any impacts strong enough to break titanium, so we are both perplexed.  

My two hypotheses: A. My harness puts pressure directly on the fused area of my back, so maybe even small lead falls (the biggest lead fall I've ever taken was with the bolt at my feet) are way higher impact than I realize? B. I was in a fender bender 11 months ago, but I don't -think- I even got whiplash from that, so could it actually have been a strong enough impact to break my screws? And then I've been climbing / bending / lifting / twisting with broken screws in my bones for an unknown number of days / weeks / months without knowing it.  

Questions:

1. Has anyone who has had lumbar fusion switched to climbing with a full body harness to aviod putting direct pressure on their lumbar spine? Does this seem necessary / worth it?

2. How am I supposed to avoid breaking more screws if I don't know why two of them broke in the first place?? Again, my bones are fully stable and my doctor has said I'm "medically interesting" but "fine to keep climbing". 

3. Is there something I'm not thinking of? I've heard of people having their hardware break, but never in a "yeah, you're actually fine, but we don't know why this happened" way. 

It' not that unusual,  Fatigue usually causes the breakage, not one event.  The key, and the reason screws were placed, was to assist in getting a bony fusion fusion.  Sometimes the screws break but allow the bones to compress and fuse.  If the bones fuse and the hardware broken or otherwise is not causing symptoms its best to leave them be.  Hope this helps.  SDSantora MD

I guess my question then is what is causing the fatigue - is climbing, especially with my harness directly over the impacted area, putting excessive strain on my hardware, or is a general "active lifestyle" likely to cause fatigue in any case? 

Maybe it doesn't really matter if my bones ARE actually fused (we'll see on the CT scan next - I've only had an x-ray so far), but I'd prefer to not have any more hardware break...

The simple act of living stresses spine hardware. Any time you sit down/stand up, twist, stretch, those implants are being repetitively torqued. Steve's assessment is correct, sometimes the hardware is removed after fusion, but usually left in situ unless otherwise causing problems (risk/benefit analysis). It's actually VERY important if the bones are fused; if they're not, there is risk for deformity or neurologic injury. Truly, this is not uncommon